1. Field of the Invention
The invention relates to a worm wheel.
2. Discussion of Background
In some electric power steering systems in which rotation of a steering assist force generating motor is transmitted to wheels via a worm gear and a worm wheel that meshes with the worm gear such that the steered angle is changed, the worm wheel is made of synthetic resin material to achieve weight reduction and noise reduction.
Conventionally, the worm wheel is formed by subjecting synthetic resin material to hobbing. However, hobbing produces a large amount of burrs, and a large number of man-hours are required to remove the burrs. Also, torque variations or abnormal noise may be caused depending on the degree of surface roughness of a machined surface. Therefore, a high processing accuracy is required, which increases the number of man-hours.
Accordingly, the worm wheel may be formed by molding. However, in each tooth of a commonly-used worm wheel, a center portion in the tooth trace direction has a smaller tooth thickness and larger tooth height than both end portions. Therefore, it is not possible to remove a molded worm wheel from a molding die in the tooth trace direction. Therefore, in order to mold a worm wheel, it is necessary to form a molding die from many components that are arranged in parallel with each other in the rotational circumferential direction of the worm wheel, and remove the molding die in the radial direction of the worm wheel. This makes the structure of a molding die considerably complex, thus increasing the manufacturing costs and reducing the molding accuracy.
Therefore, as shown in FIG. 8, Japanese Patent Application Publication No. 2001-280428 (JP 2001-280428 A) proposes that a tooth flank 101a and a tooth flank 101b of each tooth 101 of a worm wheel 100, the tooth flank 101a and the tooth flank 101b being respectively on one side and on the other side of the tooth 101 in the rotational direction, are formed of helical surface portions 101a′, 101b′ having the same shape as tooth flanks of a helical gear and concavely curved surface portions 101a″, 101b″ having a shape that follows convexly curved tooth flanks of a worm gear (not shown), respectively. In this example, the helical surface portions 101a′, 101b′ are arranged on one side of the tooth flanks 101a, 101b, respectively, in the rotational axis direction, and the concavely curved surface portions 101a″, 101b″ are arranged on the other side of the tooth flanks 101a, 101b, respectively, in the rotational axis direction. Thus, in the concavely curved surface portions 101a″, 101b″ of the respective tooth flanks 101a, 101b, the tooth height, and the distance between the tooth flanks that face each other are increased towards boundaries with the helical surface portions 101a′, 101b′. In the helical surface portions 101a′, 101b′, the tooth height and the distance between the tooth flanks that face each other are constant in the tooth trace direction. Therefore, when the worm wheel 100 is formed by molding, it is possible to remove the molded worm wheel 100 from a molding die in the tooth trace direction of the helical surface portions 101a′, 101b′ (direction of an arrow H in FIG. 8).
In the conventional worm wheel 100 described above, the helical surface portions 101a′, 101b′ are arranged on one side of each of all of the tooth flanks 101a, 101b, respectively, in the rotational axis direction, and the concavely curved surface portions 101a″, 101b″ are arranged on the other side of each of all the tooth flanks 101a, 101b, respectively, in the rotational axis direction. Therefore, a tooth thickness c of an end portion of the helical surface portion and a tooth thickness d of an end portion of concavely curved surface portion are different from each other, thereby causing variations in strength within each tooth. Moreover, because tooth flanks 102 of a worm gear are convexly curved surfaces as illustrated by a chain double-dashed line in FIG. 8, contact regions, at which the tooth flanks 102 contact the concavely curved surface portions 101a″, 101b″ are larger than contact regions at which the tooth flanks 102 contact the helical surface portions 101a′, 101b′. Therefore, when torque is transmitted to the worm wheel 100 from the worm gear that rotates in the forward direction or the reverse direction, the contact region at which the tooth flank 102 contacts the tooth 101 differs between the forward rotation and the reverse rotation. As a result, the force that is applied to the tooth face of which the strength varies differs between the forward rotation and the reverse rotation. Therefore, the smoothness of rotation transmission varies between the forward rotation and the reverse rotation, which may cause uneven abrasion and damages of the tooth flanks of the worm gear and the worm wheel 100. As a result, the service lives of the worm gear and the worm wheel 100 may be reduced.